Digital transcription system utilizing small aperture acoustical sensors

ABSTRACT

A pen transcription system and method for using the same are disclosed. The pen transcription system includes a receiver having first and second acoustical sensors mounted on a planar base and separated from one another, an EM detector, and a controller. The first and second acoustical sensors detect an acoustical signal at a first wavelength emitted by a moveable signal source. The EM detector detects an EM signal that is synchronized with the acoustical signal. The controller measures the difference in time of detection between the EM signal and the acoustical signals detected by the first and second acoustical sensors. The acoustical sensors include a detector and a housing surrounding the detector, the housing having an aperture having a maximum dimension that is less than the first wavelength divided by 6.28.

BACKGROUND OF THE INVENTION

Acoustic-based distance measuring systems have been used to compute theposition of a data entry object in a writing field for some time. Forexample, schemes that track and record the position of a pen on a whiteboard are commercially available. As the user writes on the white board,the transcription system determines the location of the pen on the boardand records the location for later use.

In such systems, a conventional marking pen of the type used with whiteboards is inserted into a housing that includes an acousticaltransmitter and an infrared transmitter. As the user writes on the whiteboard in the conventional manner, the transmitter sends a combination ofacoustical and infrared pulses. Two receivers that are separated inspace receive the signals generated by the housing. Each receivermeasures the time difference between the time of arrival of the infraredpulse and the acoustical pulse to determine the distance of the housingfrom that receiver. These distance measurements are then combined todetermine the position of the housing relative to the receivers.

Infrared is used for the light signals to avoid problems with backgroundlight in the area of use. The acoustical signals are typically in theultrasound range so that the signals are beyond the human audible range.In addition, the higher frequencies provide better spatial resolution.Each acoustical receiver is typically constructed from a microphone suchas ceramic piezo microphones, PVDF films, condenser microphones,electrets condenser microphones (ECMs), moving coil microphones etc.

Unfortunately, the sensitivity of these devices as utilized in prior artsystems is not completely omni-directional at ultrasound frequencies.The variation in angle with respect to each sensor over the range ofpositions of the pen on the surface can be relatively large. Hence,angular variation in the gain of the ultrasound receivers can lead toincreased errors due to noise and variation in the trigger point on theultrasound pulse as a function of angle. The latter type of errorresults in an error in the perceived delay time of the ultrasoundsignal, and hence, an error in the calculated distance from the sensorto the pen. In the extreme case, the microphone can have insufficientgain to detect the pen in some regions of a large writing surface. Thesegain problems can limit the size of the work surface that can betranscribed.

In addition to an omni-directional detection profile, the frequencyresponse of the sensor is also important. Even at ultrasoundfrequencies, there are narrow band background ultrasound sources thatcan interfere with the reception of the ultrasound pulse from the pen.For example, some motion detectors utilize an ultrasound signal todetect an object moving within the field of view of the motion detector.These narrow band sources can have a signal strength that is sufficientto mask the ultrasound signal from the pen in the transcription system.The transcription system pen is typically battery powered, and hence,cannot compete with a motion detector that is powered from an AC powersource and generates a signal having an amplitude that is sufficient todetect the change in frequency of the signal after the signal has beenreflected from a moving object.

SUMMARY OF THE INVENTION

The present invention includes a pen transcription system and method forusing the same. The pen transcription system includes a receiver havingfirst and second acoustical sensors mounted on a planar base andseparated from one another, an EM detector, and a controller. The firstand second acoustical sensors detect an acoustical signal emitted by amoveable signal source at a first wavelength. The EM detector detects anEM signal that is synchronized with the acoustical signal. Thecontroller measures the difference in time of detection between the EMsignal and the acoustical signals detected by the first and secondacoustical sensors. The first and second acoustical sensors each includea detector that generates an electrical signal in response to saidacoustical signal at said first wavelength and a housing surrounding thedetector, the housing having an aperture having a maximum dimension lessthan said first wavelength divided by 6.28.

In one aspect of the invention, the aperture has a plane that is definedby an axis and the acoustical sensor has a reception function that issymmetrical about the axis. The axis can be substantially parallel to orsubstantially perpendicular to the base surface depending on theparticular embodiment being implemented. The controller determines aposition for the moveable signal source.

In one aspect of the invention, the receiver is fixed relative to a worksurface on which the moveable signal source moves. The receiver can belocated on the edge of the work surface or within the work surface sothat the receiver detects signals when the moveable signal source is onat locations on all sides of the receiver. The receiver can also includea detector that is utilized by the controller to determine upon which ofthe lines connecting the first and second acoustical sensors themoveable signal source is located. In one aspect of the invention, thedetector is an EM detector that is directionally sensitive. In anotheraspect of the invention, the detector is a third acoustical sensor thatis spaced apart from the first and second detectors and located off ofthe line connecting the first and second detectors. The third acousticalsensor can also be utilized to measure the height of the moveable signalsource over the work surface.

In one aspect of the invention, the directionally sensitive EM detectorincludes two EM detectors that view different portions of the worksurface. The signals from the two EM detectors are also combined tocorrect for background EM noise that is common to the two detectors.

In a still further aspect of the invention, one of the first and secondacoustical sensors generates a signal that is proportional to theacoustic energy in the auditory acoustical band and the controlleroutputs a signal related to that signal. This signal can be utilized torecord conversations in the vicinity of the receiver together with theposition of the moveable signal source to provide a more complete recordof a presentation made on the work surface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a prior art pen transcription system.

FIG. 2 is a cross-sectional view of an acoustic sensor 30 that can beutilized in a pen transcription system.

FIG. 3 illustrates an acoustic sensor that utilizes one aspect of thepresent invention.

FIG. 4 illustrates the reception pattern in an acoustic sensor in whichD<λ/(2π).

FIG. 5 illustrates an acoustic sensor that monitors a pen as the penmoves to different positions on a work surface.

FIGS. 6A and 6B illustrate the manner in which a vertically mountedacoustic sensor provides an improved reception function.

FIG. 7 is a top view of a pen transcription system according to thepresent invention.

FIG. 8 is a top view of a pen transcription system according to anotherembodiment of the present invention.

FIG. 9 illustrates a receiver 90 that utilizes one aspect of the presentinvention.

FIG. 10 illustrates a receiver 100 that utilizes another aspect of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

The manner in which the present invention provides its advantages can bemore easily understood with reference to FIG. 1, which illustrates aprior art pen transcription system. Pen transcription system 20 operatesin conjunction with an input device 22 that has a housing that accepts astandard marking pen for writing on a work surface 21 such as a whiteboard. Pen transcription system 20 is attached to one edge of worksurface 21. Input device 22 emits both an electromagnetic (EM) signaland an acoustical signal in response to the user engaging a switch onthe input device. The acoustical signal is typically in the ultrasoundfrequency range of 30 KHz to 80 KHz. Pen transcription system 20includes an EM receiver 26 and two acoustical receivers, 24 and 25. Acontroller 27 computes the distance from input device 22 to each of theacoustical receivers by measuring the difference in arrival time betweenthe acoustical signals received at the receivers and the EM signal. Thecomputed position is forwarded to client device 28 such as a computerfor use in recording the material that was written on the work surface.

Refer now to FIG. 2, which is a cross-sectional view of an acousticsensor 30 that can be utilized in a pen transcription system. Acousticsensor 30 includes a housing 31 in which a microphone 32 is located.Housing 31 has an opening 33 having a diameter D. The reception patternfor acoustic sensor 30 depends on the relationship of D to thewavelength, λ, of the acoustical signal being detected. The receptionpattern for the case in which D>>λ/(2π) is shown at 34. It should benoted that the pattern has a main lobe that is shown at 34 and sidelobes 35. For an ultrasound signal at 40 kHz in air, λ/(2π) isapproximately 1.4 mm. Prior art devices utilize acoustic sensors inwhich D is greater than this value, and hence, have reception (andtransmission) patterns similar to that shown in FIG. 2.

This choice of aperture improves the overall sound detection byincreasing the energy that enters the cavity; however, the penalty forthis choice is a reception pattern with very high variability as afunction of angle. Define the direction normal to opening 33 as thenormal direction to the acoustic sensor. This direction is indicated bythe arrow shown at 36 in FIG. 2. Prior art pen transcription systemsutilize acoustic sensors in which the normal direction is parallel tothe work surface. Hence, the sensitivity of the acoustic sensor can varygreatly as a function of the direction of the acoustical signal relativeto the normal direction. For example, when the pen is located atposition 37, the gain can be an order of magnitude lower than the gainrealized when the pen is at location 38. This can lead to regions of thewriting surface in which the pen's position cannot be reliablydetermined, particularly in the presence of background ultrasoundsources.

Furthermore, if D>>λ/(2π), the dimensions of the cavity will be muchgreater than the wavelength of the ultrasound signal. As a result, thecavity can have a number of closely spaced resonant modes at frequenciesnear the frequencies of interest. These modes will depend on thespecific shape of the cavity. The resonant modes alter the frequencyresponse of the cavity, and hence, the frequency response of the sensorcan vary significantly as a function of the frequency of the ultrasoundsignal. Such variations can interfere with schemes that depend on abroadband signal to avoid narrow band background noise. In addition, thespecific resonant modes that are excited can vary with the angle ofincidence of the signal from the pen relative to normal 36, which canresult in further variations in the frequency response as a function ofpen location on the work surface.

Refer now to FIG. 3, which illustrates an acoustic sensor 40 thatutilizes one aspect of the present invention. In an acoustic sensoraccording to this aspect of the present invention D<λ/(2π). In thiscase, the sensitivity function is omni-directional with no side lobes asshown at 41. It should be noted that acoustic sensor 40 utilizes anacoustical sensor 42 that is substantially a pressure sensor, and hence,the reception function can be controlled by the shape and volume formedby the outer enclosure 43 and the size and thickness of the opening 33.As a result, a sensor having a size that is significantly greater than Dcan be utilized while maintaining the omni-directional receptionpattern.

Another important aspect for an acoustic sensor according to the presentinvention is the bandwidth of the reception function of the acousticsensor. The environments in which transcription systems operate ofteninclude narrow band acoustic sources in the ultrasound range. Forexample, some motion detectors utilize ultrasound sources. The signalstrength from these background sources can be significantly greater thanthat from the pen in the transcription system, and hence, some strategyis needed to differentiate the signal from the pen from that of thesebackground sources.

One scheme utilizes the bandwidth of the detected signal to make thedistinction. Motion detection transmitters utilize very narrow bandsignals; hence, a detection scheme that requires a broadband signal canbe utilized to distinguish between the background signal and the signalfrom the pen. In this aspect of the present invention, the pen generatesa signal having a broadband and the pen transcription system includes amechanism that excludes narrow band signals. For example, the ultrasoundsignal can be modulated to generate two side bands. The detection systemdepends on detecting both sidebands, and hence, can distinguish betweenthe ultrasound signal from the pen and that from a motion detector orother narrow band signal.

These schemes depend on the reception of the acoustic sensor beingsubstantially flat over a band of frequencies that is as wide as thefrequency band generated by the pen's transducer. The presence of peaksin the frequency response of the acoustic sensor created by mechanicalresonances of enclosure 43 can interfere with this type of detectionscheme. In one aspect of the present invention, an acoustic sensorutilizes a mechanical, electrical, or software filter to attenuateresonances introduced by enclosure 43 or acoustical sensor 42. Forexample, a mechanical quarter wavelength trap 44 or other baffle couldbe introduced into enclosure 43. Similarly, an analog filter 45 can beintroduced into the output of acoustical sensor 42 or the transcriptionsystem can digitize the signals from the acoustic sensors and apply anappropriate digital filter to attenuate the undesirable resonances.

Undesirable mechanical resonances in enclosure 43 can also be attenuatedor eliminated by altering the shape of enclosure 43 to eliminate closedpaths having a path length that is an integral number of wavelengths ofthe resonant frequency in question. For example, the cavity can bedesigned such that the opposite walls of the cavity are not parallel toone another.

If D<<λ/(2π), the reception function of the acoustic sensor will beomni-directional and have equal sensitivity at all angles relative tothe normal direction. Hence, reducing the size of the opening in theenclosure provides a more uniform gain as a function of the angle ofincidence of the acoustical signal with respect to the normal of theacoustic sensor. However, the amount of signal energy that is coupled tosensor 42 is also reduced if D is reduced. Hence, there is a tradeoffbetween uniformity of gain as a function of angle of incidence andsignal strength. To obtain higher overall gain while providing a gainfunction that has adequate reception over all of the angles of intereston the work surface, a D value that is characterized by a single lobereception function that has some variation in gain as a function ofangle can be necessary. Refer now to FIG. 4, which illustrates thereception pattern in an acoustic sensor in which the aperture in housing53 has a maximum dimension D<λ/(2π), but the reception pattern has somedegree of non-uniformity. The reception pattern for acoustic sensor 50is shown at 51. The gain varies as a function of angle of the incidentsignal. For example, a signal incident at angle 55 is a higher gain thanone incident at angle 56.

The degree to which this variation in reception gain as a function ofangle alters the signal strength in an acoustic sensor depends on theorientation of the acoustic sensor relative to the work surface. Refernow to FIG. 5, which illustrates an acoustic sensor 61 that monitors apen 62 as the pen moves to different positions on a work surface 60. Inthis example, the normal to acoustic sensor 61 is parallel to thesurface of work surface 60. Hence, as pen 62 moves from position 63 toposition 64 to position 65, the gain of acoustic sensor 61 decreases dueto the angular variation in the gain of acoustic sensor 61.

One aspect of the present invention is based on the observation that thereception function is only a function of the angle between the normal tothe acoustic sensor and the transmitter; hence, if the acoustic sensoris mounted such that the normal to the acoustic sensor is orthogonal tothe work surface, the variations in reception gain resulting from anon-uniform constant reception function can be substantially reduced.Refer now to FIGS. 6A and 6B, which illustrate the manner in which avertically mounted acoustic sensor provides an improved receptionfunction. FIG. 6A is a top view of acoustic sensor 61 on work surface60, and FIG. 6B is a side view of acoustic sensor 61 on work surface 60.Referring to FIG. 6B, a non-constant gain function results in the gainof acoustic sensor 61 being a function of angle 67; however, the gainwill still be a constant, independent of angle 66 for any given value ofangle 67. In the arrangement shown in FIGS. 6A and 6B, angle 67 is setby the height of the acoustical transmitter 68 in pen 62 and thedistance between the acoustic sensor 61 and pen 62. Since the height oftransmitter 68 is set by the physical dimensions of the pen, angle 67 ismainly a function of the distance from acoustic sensor 61 to the pen.Furthermore, the variation in angle 67 for various positions on the worksurface is much less than the variations of the angle 67 over the worksurface when the acoustic sensor is mounted with normal 69 parallel tothe work surface.

Refer now to FIG. 7, which is a top view of a pen transcription systemaccording to the present invention, which utilizes a vertically mountedacoustic sensor. Pen transcription system 70 tracks the position of apen 72 on a work surface 71 in a manner analogous to that describedabove. Pen 72 emits a pair of pulses, an electromagnetic pulse in theinfrared pulse together with an acoustical pulse. The pulse pairs can beemitted periodically or in response to some event such as the userpressing a button on the pen or the pen being in contact with worksurface 71. The pulse pairs are detected by a receiver 73 that includesa first acoustic sensor 74 and a second acoustic sensor 75 that areseparated by a predetermined distance and mounted such that the normalsof the acoustic sensors are perpendicular to work surface 71. Tomaximize the size of work surface 71, receiver 73 is typicallypositioned along the side of work surface 71 at a location that iscentered along one edge of work surface 71 such that the a line fromacoustic sensor 74 to acoustic sensor 75 is parallel to that edge.

Receiver 73 also includes an infrared receiver 76. Receiver 73determines the distance of pen 72 from each of the acoustic sensors bycomparing the time of arrival of the infrared pulse with the acousticalpulse received by each acoustic sensor. The position of pen 72, orinformation from which the position can be computed, is output byreceiver 73, typically to a computer that stores the information andrecreates the scene drawn by pen 72 on work surface 71 or takes otheractions based on the position motion measurements.

By adding one more infrared or acoustic sensor to the receiver, a pentranscription system having twice the working area can be constructed.Refer now to FIG. 8, which is a top view of a pen transcription systemaccording to another embodiment of the present invention that utilizesthis aspect of the present invention. Pen transcription system 80 issimilar to pen transcription system 70 discussed above in that pentranscription system 80 includes a receiver 85 having first and secondacoustic sensors shown at 87 and 88, respectively, that are mounted suchthat the normal to each acoustic sensor is perpendicular to work surface81. Receiver 85 is mounted in the center of work surface 81, and hence,receiver 85 can receive signals from both sides of receiver 85 therebyeffectively doubling the size of work surface 81 compared to worksurface 71. Receiver 85 includes two infrared receivers 82 and 83. Thedirection from which each infrared receiver can receive signals isrestricted by barrier 84 such that each infrared receiver can onlyreceive signals from one half of the work surface. Hence, the side ofthe receiver on which the pen is located can be determined.

In the arrangement shown in FIG. 8, the distance computation is carriedout using the two acoustic sensors and which ever of the infraredreceivers received the signal. However, arrangements in which a singleinfrared detector is used in the distance computations and a separatedirectional infrared receiver is used to determine on which side of thereceiver the pen is located could also be constructed. Refer now to FIG.9, which illustrates a receiver 90 that utilizes this aspect of thepresent invention. Receiver 90 includes acoustic sensors 91 and 92, andan infrared receiver 93 that are used to compute the distance from a pento each of the acoustic sensors when a pulse pair is received. Infraredreceiver 93 can receive signals from both sides of receiver 93. Aseparate directionally specific infrared detector 94 is used todetermine the location of the pen relative to receiver 90. In theembodiment shown in FIG. 9, detector 94 includes two infrared detectors95 and 96 separated by a barrier that limits the field of view of eachinfrared detector to one half the work surface on which receiver 90 isplaced.

In embodiments that include two infrared detectors that are positionedsuch that only one of the two detectors receives the infrared signalfrom the pen at any given time, the signal from the other receiver canbe used to enhance the infrared signal detection by providing ameasurement of the background infrared noise. Ambient infrared noise isintroduced by lighting and other equipment that utilize infraredsignaling such as remote controls and infrared communication links usedto transfer data between various digital devices. These sources areusually located off of the work surface and at some distance away fromthe infrared detectors. Hence, each infrared detector receives abackground signal that is the same as that received by the otherinfrared detector.

In one aspect of the invention, the signal from each infrared detectoris combined with the signal from the other infrared detector to providea signal for that detector that is corrected for the common backgroundnoise. For example, the controller in the embodiment shown in FIG. 8could form a first difference signal by subtracting the signal fromdetector 82 from the signal from detector 83 and a second differencesignal by subtracting the signal from detector 83 from the signal fromdetector 82. One of the difference signals will include a positivesignal corresponding to the infrared pulse sent by pen, and the otherwill include a corresponding negative signal. By examining thedifference signals, the controller determines the side of the receiveron which the pen is located.

The above-described embodiments utilize an arrangement in which thedirection from which the infrared signal originates is utilized todetermine on which side of the receiver the pen is located. However,embodiments that utilize an additional acoustical sensor to provide thisinformation can also be constructed. Refer now to FIG. 10, whichillustrates a receiver 100 that utilizes this aspect of the presentinvention. Receiver 100 includes acoustical sensors 91, 92, and 101 andan infrared receiver 93. The distance from a pen to each of theacoustical sensors is computed using the difference in the time ofarrival of the infrared signal and the acoustical signal from theacoustical sensor for each of the acoustical sensors. Since acousticalsensor 101 is not co-linear with acoustical sensors 91 and 92, thedistances from the acoustical sensors to the pen and the location of thepen relative to receiver 100 can be uniquely determined from the threedistance measurements.

It should also be noted that having a third acoustical detector enablesthe transcription system to determine the location of the pen inthree-dimensions. That is, the height of the pen over the work surfacecan be determined. The height information can be used to determine ifthe pen is in contact with the work surface or positioned above the worksurface. Accordingly, the controller can activate transcription onlywhen the user is actually drawing on the work surface, i.e., the pen isin contact with the work surface or within some predetermined distancefrom the surface.

As noted above, the acoustical transmitter in the pen operates at anultrasound frequency. The precise frequency depends on a number offactors. The frequency should be above the auditory range of humanbeings and domestic animals that are likely to be within range of thepen when the pen is operating. In addition, the frequency should bedifferent from that of other ultrasound transducers in the area. Suchtransducers are often used in motion sensing devices that controllighting or burglar alarms. Finally, for any given power level in thepen, higher frequency transmitters tend to have shorter ranges. Hence,pen transcription systems that must measure position over larger worksurfaces preferably utilize lower frequencies than pens that arerequired to operate over small area surfaces. In one aspect of thepresent invention, the pen includes an ultrasound transducer having afrequency between 30 KHz and 80 KHz, and more particularly between 40KHz and 80 KHz.

As noted above, the ultrasound frequency is preferably chosen to avoidbackground ultrasound sources such as those used in motion detectors. Inone aspect of the present invention, the controller measures theacoustical spectrum being received by the receivers in the absence ofthe pen to determine if there are competing ultrasound sources. In suchembodiments, the frequency of the ultrasound transducer in the pen canbe varied in some predetermined range of frequencies either continuouslyor discretely. If one or more background sources are detected, the penis set to a frequency that does not overlap with the background sourcesin question. The frequency selection can be communicated to the user bythe client device shown in FIG. 1 or by a separate frequency indicatoron the receiver. For example, receiver 100 shown in FIG. 10 couldinclude a display 110 that displays the frequency channel that the penis to utilize. The user can then set the pen frequency accordingly.

In another aspect of the present invention, at least one of theacoustical sensors is sensitive to sound in the auditory frequencyrange, i.e., 50 to 2000 Hz. The controller utilizes this detector toprovide a signal for recording the comments made by individuals withinrange of this acoustical sensor that can be included in thetranscription record with the position of the pen as a function of time.A filter can be implemented, either in hardware or software, to removesounds outside of this frequency range or a sub-range thereof that isused for the recording function.

The above-described embodiments of the present invention utilize lightin the visible or infrared region of the optical spectrum. However, asource that emits electromagnetic radiation of other wavelengths couldbe utilized provided the emitted radiation pattern is directional.Accordingly, the term “light” will be defined to include any directionalelectromagnetic radiation unless a more specific range of wavelengths isindicated.

The above-described embodiments of the present invention have beenprovided to illustrate various aspects of the invention. However, it isto be understood that different aspects of the present invention thatare shown in different specific embodiments can be combined to provideother embodiments of the present invention. In addition, variousmodifications to the present invention will become apparent to thoseskilled in the art from the foregoing description and accompanyingdrawings. Accordingly, the present invention is to be limited solely bythe scope of the following claims.

1. An apparatus comprising a receiver comprising: a base having a planarbase surface; first and second acoustical sensors that detect anacoustical signal at a first wavelength emitted by a moveable signalsource, said first and second acoustical sensors being mounted on saidbase and separated from one another; a first EM detector that detects anEM signal that is synchronized with said acoustical signal; and acontroller that measures the difference in time of detection betweensaid EM signal and said acoustical signals detected by said first andsecond acoustical sensors; wherein said first and second acousticalsensors each comprise: a detector that generates an electrical signal inresponse to an acoustical signal of said first wavelength; a housingsurrounding said detector, said housing having an aperture, saidaperture having a maximum dimension that is less than said firstwavelength divided by 6.28.
 2. The apparatus of claim 1 wherein saidaperture is defined by an axis, wherein said acoustical sensor has areception function that is symmetrical about said axis.
 3. The apparatusof claim 2 wherein said axis is substantially perpendicular to said basesurface.
 4. The apparatus of claim 2 wherein said axis is substantiallyparallel to said base surface.
 5. The apparatus of claim 1 wherein saidcontroller determines a frequency in said predetermined frequency rangeto be utilized by said moveable signal source based on a measurement ofacoustical energy as a function of frequency in said predeterminedfrequency range in the absence of said moveable signal source.
 6. Theapparatus of claim 1 wherein said controller determines a position forsaid moveable signal source.
 7. The apparatus of claim 1 wherein saidreceiver is fixed relative to a work surface on which said moveablesignal source moves.
 8. The apparatus of claim 7 wherein said receiveris located on an edge of said work surface.
 9. The apparatus of claim 7wherein said receiver is located at a point that is internal to saidwork surface, said moveable source being located on either side of aline joining said first and second acoustical sensors, and wherein saidreceiver comprises a detector that is utilized by said controller todetermine on which of said lines connecting said first and secondacoustical sensors said moveable signal source is located.
 10. Theapparatus of claim 9 wherein said detector is a second EM detector. 11.The apparatus of claim 10 wherein said controller combines signals fromsaid first and second EM detectors to generate a timing signal that iscorrected for background EM noise that is common to both of said firstand second EM detectors.
 12. The apparatus of claim 9 wherein saiddetector is a third acoustical sensor that is separated from said firstand second acoustical sensors and located off of said line connectingsaid first and second acoustical sensors.
 13. The apparatus of claim 12wherein said controller determines a height of the moveable signalsource above said work surface.
 14. The apparatus of claim 1 wherein oneof said first and second acoustical sensors generates a signal that isproportional to the acoustic energy in the auditory acoustical band andsaid controller outputs a signal related to that signal.
 15. A methodfor determining the position of a moveable signal source on a worksurface, said method comprising: providing a receiver at a fixedlocation on said work surface, said receiver comprising: first andsecond acoustical sensors that detect an acoustical signal emitted bysaid moveable signal source at a first wavelength, said first and secondacoustical sensors being mounted on said work surface and separated fromone another; and a first EM detector that detects an EM signal that issynchronized with said acoustical signal; and measuring the differencein time of detection between said EM signal and said acoustical signalsdetected by said first and second acoustical sensors; wherein said firstand second acoustical sensors each comprise: a detector that generatesan electrical signal in response to an acoustical signal in apredetermined frequency range; a housing surrounding said detector, saidhousing having an aperture, said aperture having a maximum dimensionthat is less than said first wavelength divided by 6.28.
 16. The methodof claim 15 wherein said aperture is defined by an axis, wherein saidacoustical sensor has a reception function that is symmetrical aboutsaid axis.
 17. The method of claim 16 wherein said axis is substantiallyperpendicular to said base surface.
 18. The method of claim 16 whereinsaid axis is substantially parallel to said base surface.
 19. The methodof claim 15 further comprising measuring the acoustical energy in saidpredetermined frequency range in the absence of said moveable signalsource and communicating a frequency in said predetermined frequencyrange at which said moveable signal source is to transmit saidacoustical signal based on said energy measurement.
 20. The method ofclaim 15 further comprising determining a position for said moveablesignal source on said work surface.
 21. The method of claim 15 whereinsaid receiver is located on an edge of said work surface.
 22. The methodof claim 15 wherein said receiver is located at a point that is internalto said work surface, said moveable source being located on either sideof a line joining said first and second acoustical sensors, and whereindetermining said position comprises determining on which side of a lineconnecting said first and second acoustical sensors said moveable signalsource is located.
 23. The method of claim 22 wherein determining onwhich side of said line said moveable signal source is located comprisesdetecting said EM signal in a second EM detector that provides differentsignals when said moveable source is located on different sides of saidline.
 24. The method of claim 23 further comprising combining signalsfrom said first and second EM detectors to generate a corrected EMsignal that is corrected for background EM noise that is common to bothof said first and second EM detectors.
 25. The method of claim 22wherein determining on which side of said line said moveable signalsource is located comprises detecting said acoustical signal in a thirdacoustical sensor.
 26. The method of claim 25 further comprisingdetermining the height of said moveable source above said work surface.27. The method of claim 15 further comprising recording a signal relatedto an output of one of said acoustical sensors in an auditory frequencyband.